Salt bath quenching furnace



July 25, 1950 A. F. HOLDEN SALT BATH QUENCHING FURNACE 3 Sheets-Sheet 1 Filed Dec. 10, 1946 .willllllllll 5 h 3 uaaaa"""u a EEB Imventor 3 A riemas Ffzfi/dew July 25, 1950 A. F. HOLDEN SALT BATH QUENCHING FURNACE 3 Sheets-Sheet 2 Filed Dec. 10, 1946 Snventor A N5 6070:; fJ o/aen /2/ Fig.4

Gttornegs y 1950 A. F. HOLDEN 2,516,516

SALT BATH QUENCHING FURNACE Filed Dec. 10, 1946 5 Shets-Sheet 3 Snventor Arie/270s [f /olden (Ittornegs Patented July 25, 1950 UNITED STATES PATENT OFFICE SALT BATH QUENCHING FURNACE Artemas F. Holden, New Haven, Conn.

Application December 10, 1946, Serial No. 715,161

7 Claims.

This invention relates to salt bath furnaces, and more particularly to quenching furnaces of the salt bath type. V

In the form disclosed, the present invention is directed tothe provision of a salt bath furnace which is particularly adapted for the quenching of steels to produce Inartensite or bainite or variations thereof, the purpose of this furnace being to obtain minimum distortion, and equal hardness over similar steel that is subjected to conventional quenching in'oil, water or brine.

A salt bath quenching furnace must have provision for cooling the bath in order to dissipate the heat supplied thereto by the articles quenched therein, as well as provision for heating the furnace during its idle time to replace normal heat losses .and to fuse the salt bath and bring the furnace to operating temperature when starting up. In addition, the furnace should be adapted for the maintenance of a relatively uniform temperature throughout the working area thereof in order to produce a uniform quenching action on the metals introduced therein and minimize distortion of the quenched metals. Further, the over-all temperature of the working area of the furnace, when in operation, should be held to within a range of about plus .F. or minus 5 F. in order to secure the desired quenching action.

In the form shown, this invention, which meets all of the above requirements, comprises a pot or container which is open at the top and is divided into two chambers by a partition, thus forming a working area and a sump area. A channel or trough which is disposed around the periphery of the working area communicates the top portion of the working area with the sump area. The pot is nearly filled with a salt bath which may be maintained during the idle periods in molten state at the desired operating temperature of the furnace 'by suitable heating means. The molten bath is circulated between the sump area, the working area and the connecting trough by a pump, this circulation tending to agitate the salt bath Within the working area so as to produce a more uniform temperature therein and circulate the 'hot upper layers of the salt bath through the channel and into the sump area. the bath being cooled during its flow cycle.

An object-of the present invention is to provide asalt bath quenching furnace in which the bath is positively circulated so as to maintain a nearly uniform temperature throughout'the working area of the bath.

Another object of the invention is to provide a salt .bath quenching furnace having provision for either heating or cooling of the bath in order to attain and maintain a desired operating temperature.

Another object of the invention is to provide such a furnace in which impurities introduced into the bath with articles to be quenched are promptly removed from the circulating bath to prevent contaminationof the bath.

Another object of the invention is to provide air cooling means for preventingoverheating of the bath due to heat introduced therein by the quenched articles.

A further object of the invention is to generally improve a salt bath quenching furnace of the type disclosed.

Further objects .and improvements relating to details of structure and economies .of manufacture will more-definitelyappear from the following description.

My invention is clearely defined in the appended claims. In the claims, as wellas in the description, parts are at times identified by specific names for clarity and convenience, but such nomenclature is to be understood as havin the broadest meaning consistent with the context and with the concept of my invention as distinguished from the pertinent prior art. The best form in which I have contemplatedapplying my invention is illustrated in the accompanying drawings forming part of this specification, in which:

Fig. 1 is a top plan view .of the salt bath quenching furnace, portions of the structure being broken away for clarity of illustration.

Fig. 2 is a vertical section of the furnace taken on the line 22 of Fig. .1.

Fig. 3 is another vertical section of the furnace, taken on the line.33 of Fig. 2.

Fig. 4 is another vertical section of the furnace, taken on the line 44 of Fig. 2.

Fig. 5 is .a top plan view of a .modified construction in which the heating .of the bath is accomplished by electrodes.

Fig. 6 is a vertical cross sectional viewof the :ifin odiied furnace taken on the line 6-6 of Fig. 7 is a fragmentary vertical section showing the wire baskets in the channel of the furnace.

Fig. 8 .is a, fragmentary vertical section of a modification of the furnace shown in Figs. 1 to 4 inclusive.

Referring to Figs. 1, v2 and 3. the quench furnace includes a shell I which is preferably rectangular in shape and formed of heavy gauge sheet steel. The shell Ill may be reinforced by angles ll positioned at the edges of the shell. The shell I0 is lined with a layer of refractory heat insulating material I2. The upper margin of the heat insulating material l2 may be covered with a layer of heat resistant cement composition M in order to reinforceand protect the insulating material.

A container or pot I is placed within the insulating material I2 and is preferably rectangular in shape and formed of steel or alloy plate which may be welded or otherwise formed into the shape shown. A nickel steel alloy, stainless steel, or other corrosion resistant steel may be used. A sheet metal flange or cover it, formed of material similar to that used for the shell l3, serves to cover the space between the shell and the pot l5 and also to support guard screens, to be described. I

"The pot I5 is divided into a working chamber or area l3 and a sump chamber or area 20 by a vertically positioned plate 2! which is preferably formed of metal of the same composition as the pot.' The plate 2| may be secured within the pot [5 by welding or otherwise and forms a substantially imperforate partition within the pot between the two areas.

A relatively shallow "channel or trough 22, which is open to the atmosphere, serves to communicate the upper portion of the working area IS with the sump area 20. This channel runs along the periphery of the working area I9 as shown in Fig. 1, aboutthe external walls of the pot ,l5 and along one side of the sump area 20. A portionof the wall of the pot i5 is cut away to form an inlet opening 24 extending down to the level of the bottom of the channel 22 along the end portion of the channel in order to permit passage of liquid from the upper portion of the working area into the channel, as shown in Figs. land The bottom surface of the channel is level throughout its length and at intervals has a rectangular. wire meshbasket 25 extending across the channel and upward from the bottom wall thereof to trap scale and other solid and semisolid matter contained in the circulating liquid. This basket may have an additional intermediate vertical wire screen 25 to facilitate trapping and removal of solid and semisolid matter, the baskets 25 fitting snugly within the channel 22 and being of rectangular form open at'the top and towards, the inlet opening end of the a A weir 2! is formed by cutting away a portion ofthe top wall ofithepot 15 along the side of the sump area ZEIQas shown in Fig. 3. This weir serves as the outlet forliquid flowing through the channel. r

A basket 29, generally similar in shape to baskets 26 and extending for the full length of the weir '21, is positioned within the channel 22 adjacent theweir to serveas a further. trap for solid and"s emisolid'mattera Each of the baskets or screens previously described may 'readilybe removed from thechannel 22 for removing the trapped matter, after which it may be replaced.

An inclined plate 3!! extending for the full width of the sump area and immediately beneath the weirz'l, as shown'inFig. 3 serves as a spillway to direct the liquid flowing through the weir into a thin sheet form falling through the m-idportion' ofthe sump area 20. "A horizontal plate 3| is located a short distance below the plate 30 and together therewith forms the upper and lower Walls of a chamber terminating in a narrow slot 32 extending for the full width of the sump area and immediately below the lower edge of the plate 30. An air inlet pipe 34 serves to supply air under pressure to this chamber, the air issuing in a sheet through the slot 32. A control valve 35 serves to regulate this supply of air.

A centrifugal pump 36 is supported within the sump area 20 by a mounting plate 3'! supported upon the flange IB and the top edge of the partition plate 2|. The mounting plate 37 engages an elongated shaft housing 39 of the pump, suitable clamps 40 serving to retain the housing in position on the mounting plate. An electric motor 4| is mounted on top of the housing 39 and serves to drive the pump 36. A Water cooled lower bearing 42 for the motor 4! may be used in which cool water is circulated through a chamber about the bearing in order to minimize transfer of heat to the motor.

The pump 36, which is located about midway of the height of the sump area 20, has a central inlet 34 and a peripherally located discharge line 45 projecting vertically downward from the pump. The line 45 is adapted to telescope with in a sleeve 46 of slightly greater diameter, this sleeve being in turn telescoped over vertically upwardly projecting line 41 which leads through the plate 2| and terminates in a vertically upwardly projecting leg 49 centered within the working area IS. The sleeve 46, which is not threaded, serves to facilitate withdrawal of the pump 44 from the pot l5 for maintenance or other reasons. It is unnecessary to provide a completely fluid tight seal at this joint inasmuch as any slight leakage from the discharge line merely returnsthe liquid to the sump area 2!].

Heating elements an of the electrical resistance type are positioned in the lower portion of the pot [5 within both the working area I!) and the sump area 2!}, these elements being suitably sealed in the side wall of the pot l5 and having terminals 5! located in a recess 52 formed in the insulating material l2, for connection with a suitable source of electric power. The heating elements 59 are of the ordinary immersion type the resistance element being encased in a sheath and electrically insulated from the contents of the pot 15. The heating capacity of the elements 50 is suflicient to heat a salt bath contained within the pot IE to the operating temperature of the furnace which, for the furnace herein described, is between 350 F. and 900 F. A metal grill or guard 54 is supported immediately above the heating elements 50 within the working area IS in order to protect the elements from impact of parts being quenched, this grill being supported by angles 55 secured 'to the inner faces of opposite walls of the pot l5.

A large Wire basket, best shown in Fig. 2, rests upon the grill '54 and serves to trap solid and semisolid material settling out from the salt bath within the working area [9, such materials comprising principally oxide scale and high temperature salts introduced into the furnace with the parts to be quenched. The basket 56 is rectangular in shape and has a central flanged opening 57 positioned directly above the upwardly projecting leg 49 of the line 41. Upwardly projecting bails 59, 59 are provided to facilitate removal oi the basket 56 from time to time for removal of the deposited matter therefrom, after which the basket may readily be repositioned.

A drain line 60 communicates with the bottom of the working area Hi, this line projecting through the recess52 and having a manually operated valve Bl for closing the line. The sump area 28 may be drained by the provision of a small hole 62 at the bottom of the plate or partition 2 i, this hole being of small area compared with the volume of flow through the discharge line of the pump 36 so as to permit the return of fluid therethrough to the sump at a negligible rate compared with the flow rate through-the pump discharge line 45. This arrangement permits any liquid trapped within the sump area 220 and below the pump 44 to flow into the working'area 9 for drainage through the drain linerfiil.

The channel 22 is covered by coarse wire mesh guard screens v64 which are supported upon the flange I6, these screens preferably being formed in sections, as indicated in Fig. l, for easy removal. The screens 6:. serve to protect the furnace operator from coming in contact with molten salt bath flowing through the channel 22, and at the same time permit atmospheric circulation across the top surface of the molten saltbath within the channel and permit radiation of heat from the salt bath, thus effecting a considerable cooling of the 'bath in its course through the channel.

A portion of the upper edge of the pot 15 between the working area IB and the channel 22 is cut away near the weir 21 to form an overflow opening 65 for the working area. This overflowopening will function during operation of the furnace to prevent overflow of molten salt bath from the furnace in the event that the channel 22 should become partially or completely blocked between the inlet opening 24 and the overflow opening 65. If the liquid level of molten salt within the working area reaches the level of the overflow opening 65, the liquid will flow directly into the channel 22 near its outlet end and then across the weir 21 into the sump area 20. This action of the overflow opening will become more completely apparent in connection with the description of the operation of the furnace.

In the operation of the above-described furnace, the pot it is filled with material such as a salt bath or other composition which will be completely liquid at the operating temperature of the furnace. Such a bath may be formed of sodium potassium hydroxide, preferred percentages being 50% sodium hydroxide and 50% potassium hydroxide for the ordinary range of quenching temperatures, as disclosed in my copending application Serial No. 541,281, now Patout No. 2,477,561, issued August 2, 1949 ,or nitrate-nitrite compositions.

Sufficient bath is added to the pot l so that, when in its liquid state, it will completely fill the working area l8, partially fill the channel 22 and fill the sump area 2D to a level well above the central inlet 44 of the pump :36, as indicated in the drawings. lhe furnace is initially charged with salt bath or other 'compositionin the liquid or solid state, it being understood that the levels indicated in Figs. 1 to 4, inclusive, obtain only when the pump 36 is in operation. During "idle standby periods of the furnace, it is preferable not to operate the pump 36 in order to conserve heat within the bath, at which time the channel 22 will almost completely drain and the level will become equalized between the workingarea l9 and the sumparea 20.

After placing the bath Within the pot l5, it

is brought to proper operating temperature for quenching the metal to be treated therein by means of the heating elements 5!]. This temperature varies with the particular chemical composition of the metal, as well as the type of heat treatment desired. The furnace herein described is particularly designed for the quenching of steels to form martensite, bainite or mixtures of the two structures which have been heated to a temperature in excess of 1350 F. to obtain the austenitic crystal state of the steel. Such a quenching bath may be operated at a temperature between 350 F. and 909 F. depending on the particular composition of the steel treated. The heating elements 59, acting alone, will serve to uniformly heat the salt bath within the pot l5.

Upon obtaining the desired working temperature of the bath within the working area l9, which may be determined and controlled by a pyrorneter such as a thermocouple, not shown, the bath is ready for the quenching of the metal parts therein. During this quenching, the pump 36 is maintained in operation in order to aid in maintaining the temperature of the bath within the working area I9 uniform, as well as to circulate the bath in order to cool it, and uniformly and rapidly cool the parts being quenched.

During operating periods, the bath within the working area 59 is heated by contact with the steel parts being quenched, which will result in an elevation of temperature of the bath adjacent the parts. Heated portions of the bath, being somewhat less dense than the cooler portions thereof, will tend to collect at the top of the bath as they are displaced by the cooler and more dense portions of the bath. This thermal circulation is further aided by the discharge from the upwardly-projecting leg E9 of the line 41, the discharge of which raises the level of the liquid materials within the working area [9 so as to cause the hot upper layer of the liquid to flow through the inlet opening 2c into the channel .22, the liquid level in the sump area 23 dropping accordingly. The hot liquid flows through the channel 22 where it is cooled, above mentioned, and across the weir 2'5 into the sump area 20. By projecting the stream of liquid into the sump area in a thin, flat stream flowing through the atmosphere, increased transfer of heat from. the bath eifected. This transfer may be selectively increased by passing air throu h the slot 32 by means of the air inlet pipe 34 so as to increase the cooling rate of the flowing liquid. It has been found that to maintain a relatively even temperature of the bath Within the working area It and effect suflicient cooling of the bath during its circulation, it is desirable to circulate the bath at a flow rate equal to the volume of the working area every two to four minutes. To obtain this rate of flow, two or more circulating pumps may be used operating in parallel. In addition, it has been found desirable to supply air to the air inlet pipe 34 at about 16 ounce pressure, the air supply being automatically regulated in connection with'the cooling requirement by the control valve 35.

In order to maintain a more constant temperature within the working area i9, it may be necessary to add heat to the bath even during quenching operations if the heat introduced into the bath does not exceed the heat transferred from 15 the bath. Accordingly the :pyrometer, not shown,

' furnace.

already mentioned may control in a conventional manner the electric energy supplied to the heating elements 50 to maintain the bath at the proper temperature. In addition, it has been found that better operation is obtained if the bath within the sump area is maintained at a temperature about 10 F. cooler than the bath temperature within the workin area. Accordingly, a temperature sensitive element Ill within the sump area Zll controls the operation of the automatic air control valve 35 to control the supply of cooling air in accordance with cooling' requirements.

It will be found that oxide scale and solidified and partly solidified salts from previously used high temperature salt baths will be carried into the liquid within the furnace by the parts being quenched, as well as the fixtures supporting such parts. Such matter that settles out will be caught within the large wire basket located near the bottom of the working area l9, when the pumps are not operating. Other such matter, as well as high temperature salts which start to solidify during the cooling of the liquid within the channel 22, will be retained by the baskets 26. In addition, the weir 27 and basket 29 will serve as a final trap for such matter before the flowing bath enters the sump area 28. All these baskets are readil removable for cleaning, thus aiding in preventing undue contamination of the bath.

A modified form of furnace is shown in Figs. 5 and 6. This furnace is in general similar to that already described, and is similar in its operation, the only difference being that molten salt bath. contained within this furnace is heated by electrodes positioned within the working area 19 of the furnace. A plurality of electrodes 65 may be positioned along the rear wall of the pot 15, pro-. jecting downwardly into the salt bath contained. within the working area. These electrodes are supported in the usual manner by electrode clamps 6'! and electrode supports 69, the electrode supports being mounted on the shell ill of the These electrodes may be connected to a supply source of power in any well-known manner, or, in larger furnace units, four to six electrodes may be used. The electrodes heat the salt bath contained within the pot l5 by the heating eiiect of electric current passing through the bath between the electrodes. In the furnace shown, it has been found unnecessary to position the electrodes within the sump area 20, but additional electrodes could be positioned in such area if required. In addition, the furnace could be heated by other known means such as external heating by gas.

Although a cover has not been shown or described in connection with this furnace, a conventional cover could be used and would be of some advantage during idle periods while the furnace is being maintained at quenching temperature in order to minimize the heat losses therefrom. During operation, such a cover would be detrimental in that it would decrease the desirable heat loss from the furnace.

Fig. 8 discloses a modification of the furnace shown in Figs. 1 to 4, inclusive, in which the channel 22 is made considerably deeper and somewhat wider, the level of the inlet opening 24 and weir 21 not being substantially changed. The particular advantage of such a modification is that when starting up the furnace, a better transfer of heat from the working area [9 to the solidified salt bath contained within the channel 22,

is obtained due to the fact that this solidified salt is below the level of the bath within the working area I9 so as to obtain heat directly therefrom through the wall of the pot I5. In this modification it is desirable to provide a relatively small hole H in'the wall of the pot H3 at the bottom of the channel 22 under the weir 21 in order to equalize the level of the bath in the channel 22 with the level in the remainder of the furnace during idle periods when the pump 36 is not in operation.

I claim:

1. A salt bath quench furnace comprising, an open-topped container for holding a molten salt bath, a partition extending completely across the container and dividing it into a working area and a, sump area, an open, substantially level, shallow, elongated passage in contact with the container and leading from the top portion of the working area to the sump area, and a pump communicating with the sump area and having a discharge line leading to the working area, for circulating the salt bath from the sump area to the working area and through the passage back to the sump area.

2. A salt bath. quench furnace comprising, a container adapted to hold a molten salt bath, a partition within the container dividing it into a working area and a sump area, a shallow, substantially level channel open to the atmosphere, extending around a portion of the peripher of the working area in contact with the container and providing communication between the top portion of the working area and the sump area, and a pump having an intake from the sump area and a discharge to the working area for circulating molten salt from the top of the working area through the channel into the sump and returning it to the working area.

3. A salt bath quench furnace comprising, a container adapted to hold a molten salt bath, a partition within the container dividing it into a working area and a sump area, a shallow, substantially level channel open to the atmosphere, extending around a portion of the periphery of the working area and communicating the top portion of the working area with the sump area, an overflow weir and spillway for aerating the salt bath in its passage into the sump area, and a pump for circulating the salt bath from the top of the working area through the channel into the sump area and back to the working area.

4. A salt bath quench furnace comprising, a container adapted to hold a molten salt bath, a partition within the container dividing it into a working area and a sump area, a shallow, substantially level channel open to the atmosphere, extending around a portion of the periphery of the working area and communicating the top portion of the working area with the sump area, an overflow weir and spillway for aerating the salt bath in its passage into the sump area, a pump for circulating the salt bath from the top of the working area through the channel into the sump area and back to the working area, and electric heaters within the working area for heating the salt bath.

5. A salt bath quench furnace comprising, a container adapted to hold a molten salt bath, a partition within the container dividing it into a working area and a sump area, a shallow, substantially level channel open to the atmosphere, extending around a portion of the periphery of the working area in contact with the container, and having one end communicating with the sump area, and a pump having an intake from the sump area and a discharge leading to the bottom of th working area for circulating molten salt bath from the top of the Working area through the channel into the sump area and upwardly through the working area.

6. A salt bath quench furnace comprising, a rectangular container adapted to hold a molten salt bath, a partition extending across the container generally parallel to one side and dividing it into a working area and a sump area, a shallow substantially level channel open to the atmosphere, extending around the three walls of the container adjacent the working area and in contact with the container, and having one end communicating with the working area and the other end communicating with the sump area, and a pump having an intake from the sump area and a discharge leading to the bottom of the Working area for circulating molten salt bath from the top of a working area through the channel into the sump and upwardly through the working area.

7. A salt bath quench furnace having a working zone and a separate sump zone for containing salt bath, a pump for circulating the salt bath between the working zone and the sump zone, a passage leading from the working zone to the 10 sump zone and terminating in a weir for discharging a stream of molten salt through the atmosphere into the sump zone, and means for directing a current of air against the molten salt stream to effect cooling of the bath.

ARTEMAS F. HOLDEN.

REFERENCE S CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,601,703 Patterson Sept. 28, 1926 1,803,435 Miller May 5, 1931 1,864,885 Bellis June 28, 1932 1,982,570 Cann Nov. 27, 1934 2,116,069 Hoffman et a] May 3, 1938 2,340,207 Sherrill et a] Jan. 25, 1944 FOREIGN PATENTS Number Country Date 413,514 France Aug. 11, 1910 470,939 Great Britain Aug. 25, 1937 492,066 Great Britain May 14, 1938 617,567 Germany Aug. 21, 1935 

